JOURNALOP

APPLIED

PHYSIOLOGY

Vol. 39, No. 1, July

1975.

Printed in U.S.A.

Mean

myoglobin

oxygen

exercise at maximal

tension

oxygen

during

uptake

B. J. CLARK AND R. F. COBURN Department of Physiology, University of Pennsylvania School of Medicine,

CLARK, B. J,, AND R. F. COBURN. Mean myoglobin oxygen tension during exercise at maximal oxygen uptake. J. Appl. Physiol. 39(l) : 135144. 1975.-Changes in intracellular Paz in myoglobin containing skeletal muscle during exercise were estimated in normal nonathlete subjects from measurements of shifts of CO between blood and muscle under conditions where the total body CO stores remained constant. Exercise was performed on a bicycle ergometer. In 1.52 and 6-7 min runs at Vo 2 m&x with the subject breathing 21 y0 02 , mean MbCO/HbCO increased 146 & 7 and 163 k 11% of resting values, respectively (P < 0.05). With the subjects breathing 13-14y0 02, in 1.5-2 and 6-7 min runs, To2 max fell an aver-

age of 4.3 =t 5.1% and 12.0 & 5.2y0, respectively, and mean MbCO/HbCO increased to 233 =k 18 7~ and 2 10 =t 52 y0 of resting value, respectively (P < 0.05). These findings suggest that mean myoglobin Po2 fell during exercise at ~OZ m&X , with the subiects breathing 21 y0 02 and that the decrease in mean myoglobin Po2 was greater with the subject breathing 1 3-14y0 02 . There was

considerable variability in different subjects and in some, the data were not consistent with intracellular 02 availability limiting aerobic metabolism. The data support a postulate that there are several limiting factors for the aerobic capacity, including intracellular 02 availabi.lity. intracellular

PO2 ;

carbon monoxide;

M

Mb

MHb

MbCO HbCO Mb02

AEROBIC

CAPACITY

equilibrium coefficient for the reaction Mb02 equilibrium coefficient for the reaction HbO2 carboxymyoglobin % saturation carboxyhemoglobin 70 saturation oxymoglobin % saturation

FIOz

of CO and of CO and

Pennsylvania

19174

oxyhemoglobin % saturation mean capillary 02 partial pressure myoglobin O2 partial pressure intracellular PO2 Maximal oxygen uptake (aerobic capacity) inspired % 02

A method of estimating changes in ICPo2 in skeletal muscle has been recently described, based on measurements of binding of CO to myoglobin (10, 11). The binding of CO to myoglobin is determined under chemical equilibrium conditions by the following factors MbCO

= [Pco][M&[Mb02]/[PMbo2]

Mb02 is a function of PMbo2 and n/r,, is fore, changes in MbCO are a function of Mean Pco in muscle must be eaual capillary blood which is determined by Haldane equation

a constant, thereand PCO. to mean PCO~- in ~the terms of the

POZ

Pco = [HbCO][Pc.,l/[M,,][HbOs] The partition capillary blood Eq. I and 2

aerobic metabolism

during exercise has been postulated to be limited by delivery of 02 to skeletal muscle mitochondria (5, 15, 16, 19, 22) while others have suggested that it is limited by another process so that the muscle cell is unable to utilize oxygen made available to it (13, 1820). The measurement of intracellular Po2 (ICPo2) in skeletal muscle during exercise at maximal oxygen uptake (VO, m&x) may give insight as to whether PO:! in proximity to mitochondria limits oxygen utilization or not. If delivery of 02 to muscle mitochondria is limiting 02 consumption, ICPO~ is expected to be decreased below critical levels. If aerobic metabolism is limited by a process or processes independent of oxygen delivery, it can be argued that ICPo2 will increase or be unchanged. The following abbreviations are used THE

HbOz pcoz p&3, ICPo2 VO 2 m&x

Philadelphia,

MbCO/HbCO

of CO between hemoglobin in and myoglobin can be expressed, = [Mm]

(21 mean from

[pCo,l [MbOa]/ [MHd?~boZ][Hboz]

(3)

Since Mb02 is a function of PMbog and HbO2 is a function are of pcoz, Eq. 3 shows that changes” in MbCO/HbCO due to changes in PcoZ and PMbo2. Calculations show that changes in PcoZ have only a small effect on MbCO/HbCO but that MbCO/HbCO is sensitive to changes in PMboa (10). Thus, relatively small errors are introduced in calculations of PMbos from measurements of MbCO/HbCO using assumed values of Pc9,. The sensitivity of MbCO/ in Fig. 1. Arrows HbCO to changes in PMbo2 is illustrated on this figure indicate average MbCO/HbCO values previously determined in canine skeletal or cardiac muscle biopsies (10, 11) at normal arterial Po2 and under conditions where Paoz was